125 



form of this species, its calcareous umbilical nucleus is restricted to 

 the inferior side, where it is sometimes of considerable size. It exists in 

 the youngest shells, being distinctly separated from the internal angles 

 of the respective segments by a deep groove, and often projects beyond 

 the level of the rest of the organism. As the latter increases in size 

 the nucleus also continues to receive additions, and frequently sub- 

 divides, in the older shells, into three or four large, projecting 

 tubercles. Smaller papillae also stud the surface of the contiguous 

 angles of the surrounding segments. In no instance have I suc- 

 ceeded in tracing the prolongation of pseudopodian tubes into the 

 calcareous mass, though the other portions of the organism (except- 

 ing along the supraseptal spaces) are densely crowded with them ; 

 owing to the trochoid form of the shell all the segments, including 

 even the primordial one, are brought into immediate connexion with 

 the water from which they have drawn their nutriment. 



The entire absence of pseudopodian tubes from the umbilical 

 nucleus of this organism, renders the first of the explanations, origi- 

 nally suggested in the case of 0. crispa, wholly untenable here. The 

 second supposition appears to me to be the only one capable of 

 accounting for this growth of Rotalia Beccarii. Any explanation that 

 does not meet all the phenomena bearing on the subject will not 

 satisfy the requirements of the case ; but so far as the facts hitherto 

 accumulated are concerned, the hypothesis which I have adopted in 

 the case of Amphistegina gibbosa appears sufficient. As each new 

 segment has been added to Rotalia Beccarii, the soft animal must 

 have extended its slimy form across its inferior umbilical region, and 

 thus added to the thickness of its nucleus, as well as to that of the 

 small surrounding papilla? ; and having done this, the segment has 

 probably contracted itself within the limits which it was ultimately 

 destined to occupy, and then encased itself in a special calcareous 

 cell. Whilst in Amphistegina and Polystomella such external addi- 

 tions have been made alike to both sides of the object, in R. Beccarii 

 they appear to have been chiefly, if not wholly, confined to the lower 

 surface of the organism. 



The details given in the preceding pages will have shown how de- 

 sirable it is that the soft animals of Orbiculina adunca and 0. compla- 

 nata should be examined by competent observers : any intelligent 

 individual visiting the shores of Cuba would have ample opportunities 

 of doing this. Looking at the structure of the shell of the former 

 species, and especially at the large orifices which communicate between 

 its various cavities, we cannot fail to observe that it is a reticulated, 



